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<li><a class="reference internal" href="#">Queues</a><ul>
<li><a class="reference internal" href="#queue">Queue</a></li>
<li><a class="reference internal" href="#priority-queue">Priority Queue</a></li>
<li><a class="reference internal" href="#lifo-queue">LIFO Queue</a></li>
<li><a class="reference internal" href="#exceptions">Exceptions</a></li>
<li><a class="reference internal" href="#examples">Examples</a></li>
</ul>
</li>
</ul>

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  <div class="section" id="queues">
<span id="asyncio-queues"></span><h1>Queues<a class="headerlink" href="#queues" title="Permalink to this headline">¶</a></h1>
<p><strong>Source code:</strong> <a class="reference external" href="https://github.com/python/cpython/tree/3.8/Lib/asyncio/queues.py">Lib/asyncio/queues.py</a></p>
<hr class="docutils" />
<p>asyncio queues are designed to be similar to classes of the
<a class="reference internal" href="queue.html#module-queue" title="queue: A synchronized queue class."><code class="xref py py-mod docutils literal notranslate"><span class="pre">queue</span></code></a> module.  Although asyncio queues are not thread-safe,
they are designed to be used specifically in async/await code.</p>
<p>Note that methods of asyncio queues don’t have a <em>timeout</em> parameter;
use <a class="reference internal" href="asyncio-task.html#asyncio.wait_for" title="asyncio.wait_for"><code class="xref py py-func docutils literal notranslate"><span class="pre">asyncio.wait_for()</span></code></a> function to do queue operations with a
timeout.</p>
<p>See also the <a class="reference internal" href="#examples">Examples</a> section below.</p>
<div class="section" id="queue">
<h2>Queue<a class="headerlink" href="#queue" title="Permalink to this headline">¶</a></h2>
<dl class="class">
<dt id="asyncio.Queue">
<em class="property">class </em><code class="sig-prename descclassname">asyncio.</code><code class="sig-name descname">Queue</code><span class="sig-paren">(</span><em class="sig-param">maxsize=0</em>, <em class="sig-param">*</em>, <em class="sig-param">loop=None</em><span class="sig-paren">)</span><a class="headerlink" href="#asyncio.Queue" title="Permalink to this definition">¶</a></dt>
<dd><p>A first in, first out (FIFO) queue.</p>
<p>If <em>maxsize</em> is less than or equal to zero, the queue size is
infinite.  If it is an integer greater than <code class="docutils literal notranslate"><span class="pre">0</span></code>, then
<code class="docutils literal notranslate"><span class="pre">await</span> <span class="pre">put()</span></code> blocks when the queue reaches <em>maxsize</em>
until an item is removed by <a class="reference internal" href="#asyncio.Queue.get" title="asyncio.Queue.get"><code class="xref py py-meth docutils literal notranslate"><span class="pre">get()</span></code></a>.</p>
<p>Unlike the standard library threading <a class="reference internal" href="queue.html#module-queue" title="queue: A synchronized queue class."><code class="xref py py-mod docutils literal notranslate"><span class="pre">queue</span></code></a>, the size of
the queue is always known and can be returned by calling the
<a class="reference internal" href="#asyncio.Queue.qsize" title="asyncio.Queue.qsize"><code class="xref py py-meth docutils literal notranslate"><span class="pre">qsize()</span></code></a> method.</p>
<div class="deprecated-removed">
<p><span class="versionmodified">Deprecated since version 3.8, will be removed in version 3.10: </span>The <em>loop</em> parameter.</p>
</div>
<p>This class is <a class="reference internal" href="asyncio-dev.html#asyncio-multithreading"><span class="std std-ref">not thread safe</span></a>.</p>
<dl class="attribute">
<dt id="asyncio.Queue.maxsize">
<code class="sig-name descname">maxsize</code><a class="headerlink" href="#asyncio.Queue.maxsize" title="Permalink to this definition">¶</a></dt>
<dd><p>Number of items allowed in the queue.</p>
</dd></dl>

<dl class="method">
<dt id="asyncio.Queue.empty">
<code class="sig-name descname">empty</code><span class="sig-paren">(</span><span class="sig-paren">)</span><a class="headerlink" href="#asyncio.Queue.empty" title="Permalink to this definition">¶</a></dt>
<dd><p>Return <code class="docutils literal notranslate"><span class="pre">True</span></code> if the queue is empty, <code class="docutils literal notranslate"><span class="pre">False</span></code> otherwise.</p>
</dd></dl>

<dl class="method">
<dt id="asyncio.Queue.full">
<code class="sig-name descname">full</code><span class="sig-paren">(</span><span class="sig-paren">)</span><a class="headerlink" href="#asyncio.Queue.full" title="Permalink to this definition">¶</a></dt>
<dd><p>Return <code class="docutils literal notranslate"><span class="pre">True</span></code> if there are <a class="reference internal" href="#asyncio.Queue.maxsize" title="asyncio.Queue.maxsize"><code class="xref py py-attr docutils literal notranslate"><span class="pre">maxsize</span></code></a> items in the queue.</p>
<p>If the queue was initialized with <code class="docutils literal notranslate"><span class="pre">maxsize=0</span></code> (the default),
then <a class="reference internal" href="#asyncio.Queue.full" title="asyncio.Queue.full"><code class="xref py py-meth docutils literal notranslate"><span class="pre">full()</span></code></a> never returns <code class="docutils literal notranslate"><span class="pre">True</span></code>.</p>
</dd></dl>

<dl class="method">
<dt id="asyncio.Queue.get">
<em class="property">coroutine </em><code class="sig-name descname">get</code><span class="sig-paren">(</span><span class="sig-paren">)</span><a class="headerlink" href="#asyncio.Queue.get" title="Permalink to this definition">¶</a></dt>
<dd><p>Remove and return an item from the queue. If queue is empty,
wait until an item is available.</p>
</dd></dl>

<dl class="method">
<dt id="asyncio.Queue.get_nowait">
<code class="sig-name descname">get_nowait</code><span class="sig-paren">(</span><span class="sig-paren">)</span><a class="headerlink" href="#asyncio.Queue.get_nowait" title="Permalink to this definition">¶</a></dt>
<dd><p>Return an item if one is immediately available, else raise
<a class="reference internal" href="#asyncio.QueueEmpty" title="asyncio.QueueEmpty"><code class="xref py py-exc docutils literal notranslate"><span class="pre">QueueEmpty</span></code></a>.</p>
</dd></dl>

<dl class="method">
<dt id="asyncio.Queue.join">
<em class="property">coroutine </em><code class="sig-name descname">join</code><span class="sig-paren">(</span><span class="sig-paren">)</span><a class="headerlink" href="#asyncio.Queue.join" title="Permalink to this definition">¶</a></dt>
<dd><p>Block until all items in the queue have been received and processed.</p>
<p>The count of unfinished tasks goes up whenever an item is added
to the queue. The count goes down whenever a consumer coroutine calls
<a class="reference internal" href="#asyncio.Queue.task_done" title="asyncio.Queue.task_done"><code class="xref py py-meth docutils literal notranslate"><span class="pre">task_done()</span></code></a> to indicate that the item was retrieved and all
work on it is complete.  When the count of unfinished tasks drops
to zero, <a class="reference internal" href="#asyncio.Queue.join" title="asyncio.Queue.join"><code class="xref py py-meth docutils literal notranslate"><span class="pre">join()</span></code></a> unblocks.</p>
</dd></dl>

<dl class="method">
<dt id="asyncio.Queue.put">
<em class="property">coroutine </em><code class="sig-name descname">put</code><span class="sig-paren">(</span><em class="sig-param">item</em><span class="sig-paren">)</span><a class="headerlink" href="#asyncio.Queue.put" title="Permalink to this definition">¶</a></dt>
<dd><p>Put an item into the queue. If the queue is full, wait until a
free slot is available before adding the item.</p>
</dd></dl>

<dl class="method">
<dt id="asyncio.Queue.put_nowait">
<code class="sig-name descname">put_nowait</code><span class="sig-paren">(</span><em class="sig-param">item</em><span class="sig-paren">)</span><a class="headerlink" href="#asyncio.Queue.put_nowait" title="Permalink to this definition">¶</a></dt>
<dd><p>Put an item into the queue without blocking.</p>
<p>If no free slot is immediately available, raise <a class="reference internal" href="#asyncio.QueueFull" title="asyncio.QueueFull"><code class="xref py py-exc docutils literal notranslate"><span class="pre">QueueFull</span></code></a>.</p>
</dd></dl>

<dl class="method">
<dt id="asyncio.Queue.qsize">
<code class="sig-name descname">qsize</code><span class="sig-paren">(</span><span class="sig-paren">)</span><a class="headerlink" href="#asyncio.Queue.qsize" title="Permalink to this definition">¶</a></dt>
<dd><p>Return the number of items in the queue.</p>
</dd></dl>

<dl class="method">
<dt id="asyncio.Queue.task_done">
<code class="sig-name descname">task_done</code><span class="sig-paren">(</span><span class="sig-paren">)</span><a class="headerlink" href="#asyncio.Queue.task_done" title="Permalink to this definition">¶</a></dt>
<dd><p>Indicate that a formerly enqueued task is complete.</p>
<p>Used by queue consumers. For each <a class="reference internal" href="#asyncio.Queue.get" title="asyncio.Queue.get"><code class="xref py py-meth docutils literal notranslate"><span class="pre">get()</span></code></a> used to
fetch a task, a subsequent call to <a class="reference internal" href="#asyncio.Queue.task_done" title="asyncio.Queue.task_done"><code class="xref py py-meth docutils literal notranslate"><span class="pre">task_done()</span></code></a> tells the
queue that the processing on the task is complete.</p>
<p>If a <a class="reference internal" href="#asyncio.Queue.join" title="asyncio.Queue.join"><code class="xref py py-meth docutils literal notranslate"><span class="pre">join()</span></code></a> is currently blocking, it will resume when all
items have been processed (meaning that a <a class="reference internal" href="#asyncio.Queue.task_done" title="asyncio.Queue.task_done"><code class="xref py py-meth docutils literal notranslate"><span class="pre">task_done()</span></code></a>
call was received for every item that had been <a class="reference internal" href="#asyncio.Queue.put" title="asyncio.Queue.put"><code class="xref py py-meth docutils literal notranslate"><span class="pre">put()</span></code></a>
into the queue).</p>
<p>Raises <a class="reference internal" href="exceptions.html#ValueError" title="ValueError"><code class="xref py py-exc docutils literal notranslate"><span class="pre">ValueError</span></code></a> if called more times than there were
items placed in the queue.</p>
</dd></dl>

</dd></dl>

</div>
<div class="section" id="priority-queue">
<h2>Priority Queue<a class="headerlink" href="#priority-queue" title="Permalink to this headline">¶</a></h2>
<dl class="class">
<dt id="asyncio.PriorityQueue">
<em class="property">class </em><code class="sig-prename descclassname">asyncio.</code><code class="sig-name descname">PriorityQueue</code><a class="headerlink" href="#asyncio.PriorityQueue" title="Permalink to this definition">¶</a></dt>
<dd><p>A variant of <a class="reference internal" href="#asyncio.Queue" title="asyncio.Queue"><code class="xref py py-class docutils literal notranslate"><span class="pre">Queue</span></code></a>; retrieves entries in priority order
(lowest first).</p>
<p>Entries are typically tuples of the form
<code class="docutils literal notranslate"><span class="pre">(priority_number,</span> <span class="pre">data)</span></code>.</p>
</dd></dl>

</div>
<div class="section" id="lifo-queue">
<h2>LIFO Queue<a class="headerlink" href="#lifo-queue" title="Permalink to this headline">¶</a></h2>
<dl class="class">
<dt id="asyncio.LifoQueue">
<em class="property">class </em><code class="sig-prename descclassname">asyncio.</code><code class="sig-name descname">LifoQueue</code><a class="headerlink" href="#asyncio.LifoQueue" title="Permalink to this definition">¶</a></dt>
<dd><p>A variant of <a class="reference internal" href="#asyncio.Queue" title="asyncio.Queue"><code class="xref py py-class docutils literal notranslate"><span class="pre">Queue</span></code></a> that retrieves most recently added
entries first (last in, first out).</p>
</dd></dl>

</div>
<div class="section" id="exceptions">
<h2>Exceptions<a class="headerlink" href="#exceptions" title="Permalink to this headline">¶</a></h2>
<dl class="exception">
<dt id="asyncio.QueueEmpty">
<em class="property">exception </em><code class="sig-prename descclassname">asyncio.</code><code class="sig-name descname">QueueEmpty</code><a class="headerlink" href="#asyncio.QueueEmpty" title="Permalink to this definition">¶</a></dt>
<dd><p>This exception is raised when the <a class="reference internal" href="#asyncio.Queue.get_nowait" title="asyncio.Queue.get_nowait"><code class="xref py py-meth docutils literal notranslate"><span class="pre">get_nowait()</span></code></a> method
is called on an empty queue.</p>
</dd></dl>

<dl class="exception">
<dt id="asyncio.QueueFull">
<em class="property">exception </em><code class="sig-prename descclassname">asyncio.</code><code class="sig-name descname">QueueFull</code><a class="headerlink" href="#asyncio.QueueFull" title="Permalink to this definition">¶</a></dt>
<dd><p>Exception raised when the <a class="reference internal" href="#asyncio.Queue.put_nowait" title="asyncio.Queue.put_nowait"><code class="xref py py-meth docutils literal notranslate"><span class="pre">put_nowait()</span></code></a> method is called
on a queue that has reached its <em>maxsize</em>.</p>
</dd></dl>

</div>
<div class="section" id="examples">
<h2>Examples<a class="headerlink" href="#examples" title="Permalink to this headline">¶</a></h2>
<p id="asyncio-example-queue-dist">Queues can be used to distribute workload between several
concurrent tasks:</p>
<div class="highlight-python3 notranslate"><div class="highlight"><pre><span></span><span class="kn">import</span> <span class="nn">asyncio</span>
<span class="kn">import</span> <span class="nn">random</span>
<span class="kn">import</span> <span class="nn">time</span>


<span class="k">async</span> <span class="k">def</span> <span class="nf">worker</span><span class="p">(</span><span class="n">name</span><span class="p">,</span> <span class="n">queue</span><span class="p">):</span>
    <span class="k">while</span> <span class="kc">True</span><span class="p">:</span>
        <span class="c1"># Get a &quot;work item&quot; out of the queue.</span>
        <span class="n">sleep_for</span> <span class="o">=</span> <span class="k">await</span> <span class="n">queue</span><span class="o">.</span><span class="n">get</span><span class="p">()</span>

        <span class="c1"># Sleep for the &quot;sleep_for&quot; seconds.</span>
        <span class="k">await</span> <span class="n">asyncio</span><span class="o">.</span><span class="n">sleep</span><span class="p">(</span><span class="n">sleep_for</span><span class="p">)</span>

        <span class="c1"># Notify the queue that the &quot;work item&quot; has been processed.</span>
        <span class="n">queue</span><span class="o">.</span><span class="n">task_done</span><span class="p">()</span>

        <span class="nb">print</span><span class="p">(</span><span class="sa">f</span><span class="s1">&#39;</span><span class="si">{</span><span class="n">name</span><span class="si">}</span><span class="s1"> has slept for </span><span class="si">{</span><span class="n">sleep_for</span><span class="si">:</span><span class="s1">.2f</span><span class="si">}</span><span class="s1"> seconds&#39;</span><span class="p">)</span>


<span class="k">async</span> <span class="k">def</span> <span class="nf">main</span><span class="p">():</span>
    <span class="c1"># Create a queue that we will use to store our &quot;workload&quot;.</span>
    <span class="n">queue</span> <span class="o">=</span> <span class="n">asyncio</span><span class="o">.</span><span class="n">Queue</span><span class="p">()</span>

    <span class="c1"># Generate random timings and put them into the queue.</span>
    <span class="n">total_sleep_time</span> <span class="o">=</span> <span class="mi">0</span>
    <span class="k">for</span> <span class="n">_</span> <span class="ow">in</span> <span class="nb">range</span><span class="p">(</span><span class="mi">20</span><span class="p">):</span>
        <span class="n">sleep_for</span> <span class="o">=</span> <span class="n">random</span><span class="o">.</span><span class="n">uniform</span><span class="p">(</span><span class="mf">0.05</span><span class="p">,</span> <span class="mf">1.0</span><span class="p">)</span>
        <span class="n">total_sleep_time</span> <span class="o">+=</span> <span class="n">sleep_for</span>
        <span class="n">queue</span><span class="o">.</span><span class="n">put_nowait</span><span class="p">(</span><span class="n">sleep_for</span><span class="p">)</span>

    <span class="c1"># Create three worker tasks to process the queue concurrently.</span>
    <span class="n">tasks</span> <span class="o">=</span> <span class="p">[]</span>
    <span class="k">for</span> <span class="n">i</span> <span class="ow">in</span> <span class="nb">range</span><span class="p">(</span><span class="mi">3</span><span class="p">):</span>
        <span class="n">task</span> <span class="o">=</span> <span class="n">asyncio</span><span class="o">.</span><span class="n">create_task</span><span class="p">(</span><span class="n">worker</span><span class="p">(</span><span class="sa">f</span><span class="s1">&#39;worker-</span><span class="si">{</span><span class="n">i</span><span class="si">}</span><span class="s1">&#39;</span><span class="p">,</span> <span class="n">queue</span><span class="p">))</span>
        <span class="n">tasks</span><span class="o">.</span><span class="n">append</span><span class="p">(</span><span class="n">task</span><span class="p">)</span>

    <span class="c1"># Wait until the queue is fully processed.</span>
    <span class="n">started_at</span> <span class="o">=</span> <span class="n">time</span><span class="o">.</span><span class="n">monotonic</span><span class="p">()</span>
    <span class="k">await</span> <span class="n">queue</span><span class="o">.</span><span class="n">join</span><span class="p">()</span>
    <span class="n">total_slept_for</span> <span class="o">=</span> <span class="n">time</span><span class="o">.</span><span class="n">monotonic</span><span class="p">()</span> <span class="o">-</span> <span class="n">started_at</span>

    <span class="c1"># Cancel our worker tasks.</span>
    <span class="k">for</span> <span class="n">task</span> <span class="ow">in</span> <span class="n">tasks</span><span class="p">:</span>
        <span class="n">task</span><span class="o">.</span><span class="n">cancel</span><span class="p">()</span>
    <span class="c1"># Wait until all worker tasks are cancelled.</span>
    <span class="k">await</span> <span class="n">asyncio</span><span class="o">.</span><span class="n">gather</span><span class="p">(</span><span class="o">*</span><span class="n">tasks</span><span class="p">,</span> <span class="n">return_exceptions</span><span class="o">=</span><span class="kc">True</span><span class="p">)</span>

    <span class="nb">print</span><span class="p">(</span><span class="s1">&#39;====&#39;</span><span class="p">)</span>
    <span class="nb">print</span><span class="p">(</span><span class="sa">f</span><span class="s1">&#39;3 workers slept in parallel for </span><span class="si">{</span><span class="n">total_slept_for</span><span class="si">:</span><span class="s1">.2f</span><span class="si">}</span><span class="s1"> seconds&#39;</span><span class="p">)</span>
    <span class="nb">print</span><span class="p">(</span><span class="sa">f</span><span class="s1">&#39;total expected sleep time: </span><span class="si">{</span><span class="n">total_sleep_time</span><span class="si">:</span><span class="s1">.2f</span><span class="si">}</span><span class="s1"> seconds&#39;</span><span class="p">)</span>


<span class="n">asyncio</span><span class="o">.</span><span class="n">run</span><span class="p">(</span><span class="n">main</span><span class="p">())</span>
</pre></div>
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